SHarepoint FieldDescription onchange event

I have a customer who would like to track changes made to a list in sharepoint.  They have several date fields, and want to know what the LAST date was in that field.  We were thinking a solution would be to create a new field that simply stores the last value.

I am trying to create a new field on the "EditItem" page so that when someone changes the value, another field gets auto-populated with the old value.  I was thinking of adding an onfocus, or onchange event - the issue is, I cant seem to figure out how to add the event.  I am okay with the javascript to make that work, but I dont know what part of the field to edit - I am new to sharepoint.

The field's definition looks like the following:

      <SharePoint:FieldDescription runat="server" id="ff21description{$Pos}" FieldName="_x004b_O3" ControlMode="Edit"/><SharePoint:FormField runat="server" id="ff47{$Pos}" controlmode="Edit" fieldname="_x004b_O3" __designer:bind="{ddwrt:DataBind('u',concat('ff47',$Pos),'Value','ValueChanged','ID',ddwrt:EscapeDelims(string(@ID)),'@_x004b_O3')}" />

Does anyone know how I can make it so when this field is either brought into focus, or is changed, some javascript executes?

Thanks in advance!